Led drive system for controlling an off-chip power supply

ABSTRACT

A LED drive system for controlling an off-chip power supply enables the power supply function to be located ‘off-chip’—i.e., on an IC which is separate from the IC containing the other LED drive system components. The off-chip supply provides a common line voltage for LED strings connected in series with respective current sink circuits at respective junctions, in response to a signal applied to a control input. The system includes a ‘minimum’ circuit which outputs the least of the voltages at the junctions, and an I/O pin which receives a signal that varies with the output of the minimum circuit. When the external supply&#39;s control input is coupled to the system&#39;s I/O pin, the present system controls the output of the external power supply as needed to provide a desired common line voltage.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationNo. 61/670,458 to Kraft et al., filed Jul. 11, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to series/parallel LED drive systems,and more particularly to methods of controlling the power supply whichprovides the line voltage for parallel-connected LED strings.

2. Description of the Related Art

LED lighting strategies may employ LEDs driven in series, parallel, orboth. LEDs driven in series by definition all share the same current. Ifall LEDs share the same current, ideally the brightness of the LEDs willbe matched. Some applications require a number of LEDs to be driven withmatched brightness, and so connecting the LEDs in series accomplishesthe task. A problem can arise, however, if a very large number of LEDsmust be driven in series. The series-connected LEDs are powered by aline voltage necessary to provide the necessary current; however,finding line regulators able to support the large line voltage neededfor a high LED count series string may be difficult or cost prohibitive.

LEDs may also be arranged in parallel-connected ‘strings’, each of whichis driven by a current source or (most commonly) a current sink circuit.But brightness matching between the parallel-connected LEDs is limitedby the imperfect matching of the drive circuits, which can vary widelydepending on the choice of sink implementation. A parallel LEDconfiguration does have the advantage of typically requiring a lowerline voltage than does a series configuration, which may be a benefit insome applications. Also, in some applications LEDs are connected inparallel because different currents need to be driven through the LEDs.

Due to the issues noted above, the best approach may be a compromisebetween the series and parallel solutions: a “series/parallel” solution.A cost-effective compromise employing a series/parallel solution isshown in FIG. 1. Here, each series LED string 1, 2, 3 has its ownindependent current sink circuit 4, 5, 6, but all series strings share acommon line voltage V_(line), which is provided by a voltage regulator7. The voltage on the current sink circuits (VD0, VD1, VD2) is generallyset to be equal to the maximum voltage that a string of LEDs might haveon its anode connection. This can be arranged by means of a “minimum”circuit 8, which receives the voltages on each of the current sinks andoutputs the minimum voltage of the group. An error amplifier 9 receivesthe minimum voltage and a reference voltage VD_(desired) at respectiveinputs, and provides an output V_(err) to the feedback input of voltageregulator 7 such that the current sink circuit with the minimum VDvoltage operates at a desired target voltage equal to VD_(desired).

Conventionally, the components (such as voltage regulator 7) whichgenerate common line voltage V_(line) are integrated with the otherelements of the LED drive system to form a complete LED controller IC.However, including this power supply functionality within the LEDcontroller IC may be costly and require an unacceptably large amount ofdie area.

SUMMARY OF THE INVENTION

A LED drive system for controlling an off-chip power supply is presentedwhich addresses the problems discussed above.

The present system enables the power supply function to be located‘off-chip’—i.e., on an IC which is separate from the IC containing theother LED drive system components. The off-chip power supply provides acommon line voltage for two or more LED strings that are connected inseries with respective current sink circuits at respective junctions,with each of the current sink circuits arranged to cause a desiredcurrent to be conducted by the LED string to which it is connected. Thesystem requires:

a ‘minimum’ circuit which receives the voltages at each of the junctionsat respective inputs and which outputs a ‘minimum’ voltage which isproportional to the least of the received voltages; and

an I/O pin which receives a signal that varies with the output of theminimum circuit. An external (‘off-chip’) power supply which produces anoutput voltage that varies with a signal applied to a control input canthen be used to provide the line control voltage. The supply's controlinput is coupled to the system's I/O pin, with the present systemarranged to control the output of the external power supply as needed toprovide a desired common line voltage.

The system may further include an error amplifier which receives the‘minimum’ voltage and a reference ‘desired junction voltage’ atrespective inputs and outputs a voltage that varies with the differencebetween the inputs. Then, the signal which varies with the output of theminimum circuit provided to the I/O pin can be the output of the erroramplifier. Such an arrangement might further include a variable currentsource connected to the I/O pin with is arranged to conduct a currentvia the I/O pin that varies with the output of the error amplifier. Thesystem might also provide a compensation node to which the output of theerror amplifier and the control input for the variable current sourceare connected.

The present system enables various types of off-chip power supplies tobe used to provide the common line voltage, including, for example,DC-DC converters or a PMOS transistor.

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block/schematic diagram of a known LED drive system.

FIG. 2 is a block/schematic diagram of a LED drive system forcontrolling an off-chip power supply per the present invention.

FIG. 3 is a block/schematic diagram of another possible embodiment of aLED drive system for controlling an off-chip power supply per thepresent invention.

FIG. 4 is a block/schematic diagram of another possible embodiment of aLED drive system for controlling an off-chip power supply per thepresent invention.

FIG. 5 is a block/schematic diagram of another possible embodiment of aLED drive system for controlling an off-chip power supply per thepresent invention.

FIG. 6 is a block/schematic diagram of another possible embodiment of aLED drive system for controlling an off-chip power supply per thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present LED drive system is for controlling an off-chip power supplythat supplies a common line voltage for two or more LED strings that areconnected in series with respective current sink circuits at respectivejunctions, with each of the current sink circuits arranged to cause adesired current to be conducted by the LED string to which it isconnected. The basic principles of the present system are illustrated inFIG. 2. Two or more LED strings 10, 12, 14, each typically consisting ofmultiple LEDs connected in series, are connected to respective I/O pins16, 18, 20 on an LED drive system IC 22. Each of the LED strings isconnected to a respective current sink circuit 24, 26, 28 on IC 22 atrespective junctions 30, 32, 34, and all the strings are powered by acommon line voltage V_(line). Each current sink circuit is arranged tocause a desired current to be conducted by the LED string to which it isconnected.

The system includes a ‘minimum’ circuit 36 on IC 22, which receives thevoltages at each of junctions 30, 32, 34 at respective inputs andoutputs a ‘minimum’ voltage 38 which is proportional to—and preferablyequal to—the least of the received voltages. IC 22 also includes an I/Opin 40 which receives a signal 42 that varies with the output of minimumcircuit 36; as discussed in more detail below, the output 38 of minimumcircuit 36 may undergo one or more forms of processing (44) before beingdelivered to I/O pin 40.

The LED drive system on IC 22 is arranged such that, when I/O pin 40 iscoupled (via a conductor 46) to the control input (FB) of an external‘off-chip’ power supply 50 which produces an output voltage 52 thatvaries with a signal applied to the control input, the LED drive systemcontrols the power supply's output voltage. The output 52 of externalpower supply 50 provides common line voltage V_(line). Thus, minimumcircuit 36, external power supply 50, and LED strings 10, 12, 14 form acontrol loop. In typical operation, the LED drive system on IC 22 variesthe signal at I/O pin 40 as needed to cause external power supply 50 toprovide the common line voltage V_(line) necessary to maintain minimumvoltage 38 at a level necessary to ensure that a minimum amount ofvoltage headroom is provided for all of current sinks 24, 26, 28.

2When so arranged, with the LED drive system controlling an externalpower supply which resides off-chip, readily-available off-the-shelfpower supply devices may be used to provide V_(line). As such, thisfunctionality can be omitted from IC 22, reducing the cost, circuitcomplexity and required die area of the LED drive system IC.

As shown in FIG. 3, I/O pin 40 is suitably designated as a ‘feedback’I/O pin (FB OUT), with the present LED drive system further comprisingan error amplifier 60 which receives ‘minimum’ voltage 38 and areference ‘desired junction voltage’ Vref at respective inputs and whichoutputs a voltage 62 that varies with the difference between its inputs.Here, the signal 42 which varies with the output of minimum circuit 36and is provided to I/O pin FB OUT is the output 62 of error amplifier60.

Though output 62 of error amplifier 60 might be directly connected toI/O pin FB OUT or be otherwise processed, it is preferred that output 62be connected to a variable current source 64, which is connected to FBOUT and arranged to conduct a current via FB OUT that varies with theoutput of the error amplifier. In this way, a signal is provided at FBOUT which is suitable for controlling a number of different types ofexternal power supplies.

The LED drive system on IC 22 might also include a compensation nodeCOMP, to which compensation components 66 might be connected. The output62 of error amplifier 60 (and the control input of variable currentsource 64) can then be connected to compensation node 66, withcomponents 66 providing stability to the control loop formed by theexternal power supply, LED strings, minimum circuit and error amplifier.

The present system provides a means of controlling a variety of powerstage types that can be used to generate V_(line). One possible externalpower supply 50 is simply an off-chip transistor; an exemplaryembodiment is shown in FIG. 3. Here, the current circuit of thetransistor 70 is connected between an input voltage Vin and an outputnode OUT, with the control input of the transistor coupled to I/O pin FBOUT. The voltage at output node OUT is common line voltage V_(line). Inthis example, transistor 70 is a PMOS FET, with its drain-source circuitconnected between OUT and Vin, and its gate input coupled to I/O pin FBOUT; other transistor types could also be used.

In operation, a control loop is formed by transistor 70, LED strings 10,12, 14, minimum circuit 36, error amplifier 60 and variable currentsource 64. The LED drive system on IC 22 varies the signal 46 at I/O pinFB OUT as needed to cause transistor 70 to produce a common line voltageV_(line) which makes the ‘minimum’ voltage 38 equal to desired junctionvoltage Vref. When there is a large error between ‘minimum’ voltage 38and Vref, current source 64 is driven to conduct more current. Thisdecreases the gate voltage of PMOS FET 70, which decreases itsresistance and causes the voltage at output node OUT—and thusV_(line)—to increase as needed to drive ‘minimum’ voltage 38 towardsVref.

Another possible external power supply 50 could be an off-chip DC-DCconverter 78; one possible embodiment is shown in FIG. 4. Here, theDC-DC converter has a transformer T1 with a primary side and a secondaryside, with the secondary side coupled to an output node OUT. The DC-DCconverter may also include an optocoupler 80 (which may be a part of theconverter or a separate off-chip component) having an input 82 which iscoupled to feedback I/O pin FB OUT and an output 84 which is connectedto the control input FB of the DC-DC converter's controller 86. Thecontroller drives the primary side of transformer T1 to produce anoutput voltage at output node OUT; this voltage is employed as V_(line).

As before, in typical operation, the LED drive system on IC 22 variesthe signal 46 at I/O pin FB OUT as needed to cause power supply 50 toproduce a common line voltage V_(line) which makes the ‘minimum’ voltage38 equal to desired junction voltage Vref. When there is a large errorbetween ‘minimum’ voltage 38 and Vref, current source 64 is driven toconduct more current. As the FB OUT current increases, the voltage onoptocoupler 80 decreases, which causes the controller to increase thecurrent conducted by the primary side of transformer T1, resulting inthe voltage at output node OUT—and thus V_(line)—increasing as needed todrive ‘minimum’ voltage 38 towards Vref.

Another possible embodiment is shown in FIG. 5. In this example,external power supply 50 is an off-chip DC-DC converter 90 which has itsown error amplifier 92. Since converter 90 has its own error amplifier,it is not necessary to produce an error amplifier output from within theLED drive system on IC 22. Instead, the output 38 of minimum circuit 36is provided to I/O pin 40; if necessary, output 38 can be scaled with ascaling circuit 94 prior to its being provided to I/O pin 40. Erroramplifier 92 receives a reference voltage REF at one input and iscoupled to I/O pin 40 at its other input; the signal at I/O pin 40 canbe divided down with a divider 96 if needed to match REF. Erroramplifier 92 thus produces an output OUT which varies with thedifference between ‘minimum’ voltage 38 and REF, with the voltage atoutput node OUT being common line voltage V_(line). The LED drive systemon IC 22 varies the signal 46 at I/O pin 40 as needed to produce acommon line voltage V_(line) which makes the ‘minimum’ voltage equal tothe desired junction voltage. DC-DC converter 90 could be, for example,a boost converter, a buck converter, or a low dropout (LDO) voltageregulator.

As shown in the embodiment shown in FIG. 6, the present LED drive systemmay further comprise an ‘enable’ I/O pin EN on IC 22 which provides asignal 100 that indicates whether the external power supply should beactive or shut down. Then, if external power supply 50 includes anenable input EN that is coupled to the I/O pin EN, and is arranged toproduce a non-zero output voltage when the signal on the enable I/O pinindicates that external power supply 50 should be active and to shutdown when the signal on the enable I/O pin indicates that the externalpower supply should be shut down, then the LED drive system of IC 22controls the state of external power supply 50. The enable signal 100 issuitably produced by a processing module 102 on IC 22.

The present LED drive system may also include a ‘voltage sense’ I/O pinSENSE which receives a signal 104 from external power supply 50 thatvaries with its output voltage. A comparison circuit (not shown),suitably located within processing module 102, is arranged to comparesignal 104 with a limit voltage, and to toggle an output when thecomparison circuit indicates that the output voltage is greater than thelimit voltage. The output that is toggled by the comparison circuit issuitably the EN signal 100 which operates to shut down external powersupply 50. This mechanism might be used to shut down external powersupply 50 in the event of an overvoltage condition that occurs due tosome system fault. Additional inputs 106 might also be used to toggle ENsignal 100 when there is a need to shut down external power supply 50.

The embodiments shown are merely exemplary, and are used only toillustrate how the present LED drive system can be used with a varietyof power stage types, including stand-alone off-the-shelf boost and buckconverters. It is only necessary that the external power supply bearranged to produce an output voltage that varies with a signal appliedto its control input, such that connecting the control input to an I/Opin on the LED drive system IC as described herein enables the LED drivesystem IC to control the output voltage produced by external powersupply 50.

One of the advantages of this approach is that any power stage can beused with the LED control stage. By separating the power stage from theLED drive system, each of the separate ICs can be optimized.

The embodiments of the invention described herein are exemplary andnumerous modifications, variations and rearrangements can be readilyenvisioned to achieve substantially equivalent results, all of which areintended to be embraced within the spirit and scope of the invention asdefined in the appended claims.

1. An LED drive system which resides on a first integrated circuit (IC)and is arranged to control an off-chip power supply that supplies acommon line voltage for two or more LED strings that are connected inseries with respective current sink circuits at respective junctions,each of said current sink circuits arranged to cause a desired currentto be conducted by the LED string to which it is connected, said LEDdrive system comprising: a minimum circuit which receives the voltagesat each of said junctions at respective inputs and which outputs aminimum voltage which is proportional to the least of the receivedvoltages; and an I/O pin on said first IC which receives a signal thatvaries with the output of said minimum circuit, wherein said I/O pin isa feedback I/O pin; said LED drive system arranged such that, when saidI/O pin is coupled to the control input of an external power supplywhich produces an output voltage that varies with a signal applied tosaid control input, said LED drive system controls said power supply'soutput voltage, the output voltage of said external power supplyproviding said common line voltage; and said LED drive system furtherarranged such that, when said external power supply which produces anoutput voltage that varies with a signal applied to a control input is aDC-DC converter having a transformer with a primary side and a secondaryside, with said secondary side coupled to an output node, and anoptocoupler having an input which is coupled to said feedback I/O pinand with the voltage at said output node being said common line voltage,said LED drive system varies the signal at said feedback I/O pin asneeded to produce a common line voltage which makes said minimum voltageequal to said desired junction voltage.
 2. The LED drive system of claim1, further comprising an error amplifier which receives said minimumvoltage and a reference desired junction voltage at respective inputsand which outputs a voltage that varies with the difference between saidinputs, said signal which varies with the output of said minimum circuitprovided to said feedback I/O pin being the output of said erroramplifier.
 3. The LED drive system of claim 2, further comprising avariable current source connected to said feedback I/O pin and arrangedto conduct a current via said feedback I/O pin that varies with theoutput of said error amplifier.
 4. The LED drive system of claim 3,further comprising a compensation node, said variable current sourcearranged to conduct a current that varies with a signal applied to acontrol input, the output of said error amplifier and said control inputconnected to said compensation node.
 5. The LED drive system of claim 4,further comprising one or compensation components connected to saidcompensation node.
 6. The LED drive system of claim 1, furthercomprising a DC-DC converter having a transformer with a primary sideand a secondary side, with said secondary side connected to an outputnode, and an optocoupler having an input which is coupled to saidfeedback I/O pin and with the voltage at said output node being saidcommon line voltage, said DC-DC converter being said external powersupply.
 7. The LED drive system of claim 6, wherein said DC-DC converteris a boost converter, a buck converter, or a low dropout voltageregulator.
 8. An LED drive system which resides on a first integratedcircuit (IC) and is arranged to control an off-chip power supply thatsupplies a common line voltage for two or more LED strings that areconnected in series with respective current sink circuits at respectivejunctions, each of said current sink circuits arranged to cause adesired current to be conducted by the LED string to which it isconnected, comprising: a minimum circuit which receives the voltages ateach of said junctions at respective inputs and which outputs a minimumvoltage which is proportional to the least of the received voltages; afirst I/O pin on said first IC which receives a signal that varies withthe output of said minimum circuit, wherein said I/O pin is a feedbackI/O pin; an error amplifier which receives said minimum voltage and areference desired junction voltage at respective inputs and whichoutputs a voltage that varies with the difference between said inputs;and a variable current source connected to said feedback I/O pin andarranged to conduct a current via said feedback I/O pin that varies withthe output of said error amplifier; said LED drive system arranged suchthat, when said feedback I/O pin is coupled to the control input of anexternal power supply which produces an output voltage that varies witha signal applied to said control input, said LED drive system controlssaid power supply's output voltage, the output voltage of said externalpower supply providing said common line voltage.
 9. The LED drive systemof claim 8, wherein, said signal which varies with the output of saidminimum circuit provided to said feedback I/O pin being the output ofsaid error amplifier.
 10. The LED drive system of claim 1, said LEDdrive system further arranged such that, when said external power supplywhich produces an output voltage that varies with a signal applied to acontrol input is a DC-DC converter having an error amplifier which iscoupled to said I/O pin and to a reference voltage at respective inputsand which produces an output voltage at an output node that varies withthe difference between said ‘minimum’ voltage and said referencevoltage, the voltage at said output node being said common line voltage,said LED drive system varies the signal at said I/O pin as needed toproduce a common line voltage which makes said ‘minimum’ voltage equalto said desired junction voltage.
 11. The LED drive system of claim 10,further comprising a DC-DC converter having an error amplifier which iscoupled to said I/O pin and to a reference voltage at respective inputsand which produces an output voltage at an output node that varies withthe difference between said ‘minimum’ voltage and said referencevoltage, the voltage at said output node being said common line voltage,said DC-DC converter being said external power supply.
 12. The LED drivesystem of claim 11, wherein said DC-DC converter is a boost converter, abuck converter, or a low dropout voltage regulator.